An argon laser photocoagulator has been developed during the past four years which presently offers good endoscopic control of massive gastric bleeding. Because of the deeper penetration depth of Nd:YAG laser radiation into normal or ulcerated mucosa the possibility for better endoscopic control of massive G.I. bleeding exists in this wavelength. The objectives of this portion of the proposal are to: (a) develop a reliable high-power Nd:YAG single fiber waveguide catheter capable of delivering greater than 75 W of 1.06 micron radiation for endoscopic coagulation; (b) development of a CO2 gas jet assist for the Nd:YAG delivery system; (c) comparative evaluation of efficacy and safety at this wavelength in a standard animal ulcer model utilizing acute and chronic histology; (d) assess the benefit risk ratio for potential application to clinical trials. A prototype automated thermal probe has been developed which has proven highly effective in staunching massive gastric hemorrhage. This unit is not presently refined to the point of assuring shallow injury to the gastric wall. With rapid heating and cooling of the thermal probe the depth of damage should be much more predictable. By using microcircuit fabrication techniques such a probe can be developed. The objectives of this portion of the proposal are to: (a) develop several prototype automated thermal probes with high wattage (greater than 10W) dissipation capacity and low thermal mass; (b) test these probes for reliability, temporal response, and heat transfer capacity; (c) test the prototypes for efficacy in our standard animal ulcer model, and assess safety with acute and chronic histology; (d) develop a CO2 gas jet assisted probe for animal testing; (e) determine benefit/risk ratios and assess merits for controlled clinical trials.